Erythropoietin (EPO) stimulates the proliferation and differentiation of erythroid cells and is believed to be the primary regulator of erythropoiesis. The action of erythropoietin is initiated by its binding to a specific cell surface receptor followed by receptor-mediated endocytosis. However, the molecular details regarding the manner in which erythropoietin binding to its receptor stimulates erythropoiesis remains unknown. We have isolated, characterized and sequenced a genomic clone of the human erythropoietin receptor from a placental genomic library. The coding region spans about 6.5kb with seven intervening sequences ranging in size from 81 bp to 2.1 kb. The human EPO receptor contains a palindromic sequence 5' of the translated region which consists of an almost perfect inverted repeat of 12 nucleotides (CAGCTGC(G/C)TCCG) centered about G at -92 from the first codon. An inverted SP1 binding site (CCGCCC) and an inverted GATA-1 binding site (TTATCT) are located at positions -151 and -179, respectively, and CACCC sequences are located at -585 and further upstream. No TATA or CAAT sequences are in this 5' flanking region. However, this region as far as -275 has a 72% GC content compared with an overall GC content of 56%. A 1Kb Bam H1 fragment of the human EPO receptor containing 700 bp of sequences 5' of the coding region was transcribed in an in vitro transcription assay; initiation of transcription appeared to be around 132 just down stream from the inverted SP1 site at -151. T1 analysis of human EPO receptor mRNA also maps the site of transcription initiation to this region. Within 180 nucleotides 5' to the first exon are three regions with 70% greater homology with the murine EPO receptor. We are currently in the process of detailed analysis of promoter elements and other cis-acting sequences which may provide information regarding possible trans-acting factors which regulate the developmental expression of the EPO receptor and more generally the developmental response to erythropoietin. The study of this gene including its similarities with other hematopoietic cytokine receptors should help elucidate aspects of the transcriptional and possible translational control of the EPO receptor in human erythroid cells and thus its role in signal transduction and erythroid differentiation. We have also established several transgenic mice expressing the EPO receptor and will examine the effects on the animal's phenotype.